1. Field of the Invention
The present invention relates to a signal branching method applied to a protection switching equipment provided in a drop-insert station of a transmission system.
2. Description of the Related Art
A transmission system is for transmitting a users signal or data, which will be represented by a users signal hereinafter, through a transmission line starting from one end station to another end station and providing a plurality of drop-insert stations between the end stations in series. The transmission line consists of a plurality of work lines through which the users signals are transmitted and a protection line for protecting the users signal from meeting with failure of transmission due to an accident such as disconnection of the work line.
The protection line is also used for a liaison communication among the end and the drop-insert stations. However, in order to detect the disconnection of the protection line and to avoid producing alarm for envoying that a work line has a disconnection trouble to other stations, a protection signal is constantly transmitted through the protection line. Formerly, a dummy signal was expressly generated as the protection signal at the end stations and transmitted through the protection line. However, recently, one of the users signals transmitted through one of the work lines has come to be used instead of the dummy signal. This is for decreasing the size and the costs of each END station.
FIG. 1 is a block diagram of a transmission system of the related art. The transmission system consists of a transmission line, END stations placed at the both ends of the transmission line and a plurality of drop-insert stations placed between the END stations in series on the transmission line. The transmission line includes a plurality of the work lines (WKs 1, 2, - - - n) and a protection line (PT). FIG. 1 shows a case that the users signals and a protection signal are transmitted from an end station (END STN) 100 to a drop-insert station (DI STN) 200 adjacent to END STN 100 through a plurality of work lines WK1, WK2, - - - WKn and a protection line (PT) respectively.
In FIG. 1, the END STN 100 includes a line terminal equipment (LTE) 120 for transmitting the protection signal to DI STN 200 through PT and for transmitting and receiving the liaison signals between END STN 100 and DI STN 200 respectively, and END STN 100 includes LTEs 121, 122, - - - for transmitting the users signals to DI STN 200 through WK1, WK2, - - - respectively. The DI STN 200 includes LTEs 220 for receiving the protection signal from END STN 100 and for transmitting and receiving the liaison signals transferring between END STN 100 and DI STN 200, and DI STN 200 also includes LTEs 221, 222, - - - for receiving the users signals sent from END STN 100 through WK1, WK2, - - -, and DI STN 200 further includes LTEs 230, 231, 232, - - - at the ends of PT, WK1, WK2, - - - between DI STN 200 and a succeeding DI STN same as LTEs 120, 121, - - - respectively.
When the protection signal and the users signals are transmitted from or received at those LTEs, each of these signals always passes through a protection switching equipment (PSW). The PSW is for switching a transmitting route of a users signal to PT when WK for transmitting the users signal falls into a disconnection trouble. In FIG. 1, END STN 100 has PSW 101 and DI STN 200 has PSWs 201 and 202, PSW 201 is a receiver side PSW (Rx-PSW) and PSW 202 is a transmitter side PSW (Tx-PSW). Therefore, when the transmission line is normal, which means that there is no disconnection trouble in PT and WKs, the users signals given to END STN 100 are sent to LTEs 121, 122, - - - through a switch for WK1 (SW(WK1)), a switch for WK2 (SW(WK2)), - - - of PSW 101 in END STN 100, and the users signals arrived at LTEs 221, 222, - - - in DI STN 200 through WK1, WK2, - - - are sent to LTEs 231, 232, - - - in DI STN 200 through SWs(WK1) in Rx-PSW 201 and Tx-PSW 202, SWs(WK2) in Rx-PSW 201 and Tx-PSW 202 respectively, and the protection signal given to END STN 100 is sent to LTE 120 through a switch for PT (SW(PT)) in PSW 101, and the protection signal arrived at LTE 220 in DI STN 200 through PT is sent to LTE 230 in DI STN 200 through SWs(PT) in Rx-PSW 201 and Tx-PSW 202.
The details will be explained later, however, FIG. 1 shows a case of using a users signal as the protection signal, so that in FIG. 1, a users signal being to be transmitted through WK1 branches at a hybrid circuit (H) in SW(WK1) to SW(PT) in PSW 101. Therefore, the users signals are transmitted through WK1, WK2, - - - and WKn as shown by thick solid lines and the one of the users signal, for instance, the users signal to be transmitted through WK1 is used as the protection signal as shown by a thick solid line marked by PT. However, when WK1 has the disconnection trouble as shown by a mark "X" in FIG. 1, PSWs in END STN 100 and DI STN 200 operates as follows: SW(PT) in PSW 101 operates so that the users signal to be transmitted through WK1 is transmitted through PT between END STN 100 and DI STN 200; and in PSW of DI STN 200, SWs(PT) in Rx-PSW 201 and Tx-PSW 202, SWs(WK1) in Rx-PSW 201 and Tx-PSW 202 operate so that the users signal transmitted to DI STN 200 through PT is sent to SW(PT) in Tx-PSW 202 through SW(PT) and SW(WK1) in Rx-PSW 201 and SW(WK1) in Tx-PSW 202 as shown by a dotted thick line in FIG. 1. Because, though the users signal to be transmitted through WK1 has been transmitted from END STN 100 to DI STN 200 through PT, the users signal must be transmitted from DI STN 200 to the succeeding DI STN through WK1 but at the same time the users signal must branch to PT. This branching is made by H in Tx-PSW 202 of DI STN 200. The control performed thus in PSWs in END STN 100 and DI STN 200 has been too complex. The complexity will be explained in reference to FIG. 3 which shows another transmission system of the prior art.
Before explaining FIG. 3, a prior art transmission system including a dummy generator for generating a dummy signal as the protection signal will be explained in reference to FIG. 2. In FIG. 2, the same reference numeral or symbol as in FIG. 1 designates the same unit as in FIG. 1. Same as in FIG. 1, FIG. 2 shows a case of transmitting the users signals from END STN 100 to DI STN 200, and FIG. 2 depicts only one users signal transmitted through WK1. Generally, PSW has a control unit such that PSW in an END STN has a control unit called a master control unit and PSW in a DI STN has a control unit called a slave control unit. The master control unit controls not only the own PSW in the END STN but also the slave control units in DI STNs over the transmission system, and control signals required to be sent among the master control unit and the slave control units are transferred through PTs provided between END STN and DI STN and between END STNs. In FIG. 2, PSW 11' in END STN 100 includes a master control unit (MASTER CONT) 150' and PSW 21" in DI STN 200 has a slave control unit (SLAVE CONT) 210". The END STN 100 includes a dummy generator (DUMMY GEN) 130 for generating a dummy signal to be transmitted through PT as the protection signal when no disconnection trouble occurs in WKs. An SW(PT) 110' in PSW 11" includes two relays (RLY(11) and RLY(12)) and an SW(WK1) 111' in PSW 11' includes also two relays (RLY(13) and RLY(14)) so that the relays make connection as shown by a dotted line respectively when WKs have no disconnection trouble. Same as the above, PSW 21" in DI STN 200 has SW(PT) 211' including RLY(21) and RLY(22) and has SW(WK1) 212' including RLY(23) and RLY(24), making the relays connect circuits as shown by a dotted line respectively when WKs have no disconnection trouble. Therefore, when WKs have no disconnection trouble, the dummy singal is transmitted through PTs and the users signals are transmitted through WKs (only WK1 is depicted in FIG. 2) in the transmission system. However, when a disconnection trouble occurs, for example, a disconnection trouble occurs on WK1 as marked by X in the figure, LTE 221 in DI STN 200 detects that no users signal receives thereat and produces a detection signal for envoying the occurrence of the disconnection trouble to be sent to SLAVE CONT 210". When SLAVE CONT 210" receives the detection signal, SLAVE CONT 210" sends the detection signal to MASTER CONT 150' through LTE 220, PT between DI STN 200 and END STN 100 and LTE 120. When MASTER CONT 150' receives the detection signal, MASTER CONT 150' controls the own SW(PT) 110' and SW(WK1) 111' so that RLYs(11), (12), (13) and (14) make connection of circuits as shown in FIG. 2. After that, MASTER CONT 150' sends a command signal to SLAVE CONT 210" through LTE 120, PT and LTE 220. After receiving the command signal, SLAVE CONT 210" controls SW(PT) 211' and SW(WK1) 212" so that RLYs(21, 22, 23 and 24) make connection of circuits as shown in FIG. 2. (In each switch mentioned above has a hard soft interface (H/S INT) for making the hardware (relay) of each switch operate in accordance with a control signal sent from the control unit.) As a result, in END STN 100, the dummy signal from DUMMY GEN 130 is sent to LTE 121 but not sent to DI STN 21" because of the disconnection trouble, and the users signal to be sent to DI STN 200 through WK1 is switched to LTE 120 so as to be sent to DI STN 200 through PT, and in DI STN 200, the users signal sent to LTE 220 is switched to LTE 231 and sent to the succeeding DI STN through WK1 and an alarm signal produced in LTE 221 for envoying the occurrence of the disconnection trouble to the other stations is sent to LTE 230 through RLY(24) and RLY(21) so as to be sent to the succeeding DI STN through PT between DI STN 200 and the succeeding DI STN.
Different from FIG. 2, FIG. 3 is a case of using a users signal as the protection signal instead of the dummy signal. (The branch circuit of the users signal is not depicted in the figure.) In FIG. 3, the same reference numeral or symbol as in FIG. 2 designates the same unit as in FIG. 2, and FIG. 3 is also a case of transmitting the users signal from END STN to DI STN. Therefore, in FIG. 3, one of the users signals to be sent through WKs is sent to RLY(15) in SW(PT) 110 in PSW 11' of END STN 100. When no disconnection trouble occurs on WKs, CONT 150 controls SW(PT) 110 and SW(WK1) 111 so that the relays in PSW 11' of END STN 100 and in PSW 21' of DI STN 200 make connection of circuits as shown by a dotted line in each relay. As a result, a users signal used as the protection signal is transmitted through PT and the users signal to be sent through WK1 is transmitted through WK1 straight from END STN 100 to the succeeding DI STN through DI STN 200 as shown in FIG. 3.
When WK1 has a disconnection trouble in FIG. 3, the trouble is detected at LTE 221 in DI STN 200 and a detection signal is output from LTE 221 to SLAVE CONT 210' in PSW 21' of DI STN 200. Then, same as described in reference to FIG. 2, SLAVE CONT 210' sends the detection signal to MASTER CONT 150 in PSW 11' of END STN 100, through PT. When MASTER CONT 150 receives the detection signal, MASTER CONT 150 controls SW(PT) 110 and SW(WK1) 111 so that the relays in the switches make connection of circuits as shown in FIG. 3. After controlling the relays in PSW 11', MASTER CONT 150 sends a command signal to SLAVE CONT 210' in PSW 21' of DI STN 200. When SLAVE CONT 210' receives the command signal from MASTER CONT 150, SLAVE CONT 210' controls SWs(PT) 211'(a) and 211'(b) and SWs(WK1) 212(a) and 212(b) so that the relays in the switches make contact as shown in FIG. 3.
As a result, in END STN 100, a users signal to be sent through WK1 branches at H1 in SW(WK1) 111 and is relayed to LTE 120 so that the users signal is sent to DI STN 200 through PT instead of WK1, and the protection signal (one of the users signals) is stopped from being sent to DI STN 200 by RLY(15) in SW(PT) 110 of PSW 11'. In DI STN 200, the alarm signal output from LTE 221 is stopped from being sent to LTE 2 30 by RLY(26) in SW(WK1) 212(a), and the users signal sent from END STN 100 through PT is received at LTE 220 and switched to LTE 231 by RLY(25) in SW(PT) 211'(a) and RLYs(26 and 27) in SW(WK1) 212(a). The users signal switched to LTE 231 branches to SW(PT) 211'(b) by H2 in SW(WK1) 212b and is relayed to LTE 230 by RLY(28) in SW(PT) 211'(b) so as to be used as a protection signal transmitted from DI STN 200 to the succeeding DI STN through PT.
Comparing the transmission systems in FIGS. 2 and 3, the system in FIG. 3 is superior to the system in FIG. 2. Because, the DUMMY GEN is not necessary to be provided to END STN 100 in FIG. 3, which results in reducing the size and costs of END STN 100.
In FIG. 3, MASTER CONT 150 and SLAVE CONT 210' have a central processing unit (CPU) respectively. Using the CPU of MASTER CONT 150, MASTER CONT 150 controls not only the switches in PSW 11' of END STN 100 but also SLAVE CONT 210' by sending the command signal from MASTER CONT 150 to SLAVE CONT 210', and using the CPU in SLAVE CONT 210', SLAVE CONT 210' controls the switches in PSW 21' of DI STN 200.
FIG. 4(a) shows a control flow chart of MASTER CONT 150 in FIG. 3. Because of software previously provided to CPU in MASTER CONT 150, MASTER CONT 150 operates as follows:
periodically scans (SCAN START) whether there is a disconnection trouble in WK (ALM?);
when MASTER CONT 150 receives a detection signal envoying the occurrence of the disconnection trouble, MASTER CONT 150 searches where the detection signal comes from and recognizes that the signal comes from WK1 for instance (WHERE? WK1);
then, MASTER CONT 150 controls RLYs(15) and (16) in PSW 11' of END STN 100 so as to make connection of circuits as shown in FIG. 3 and sends a command signal to SLAVE CONT 210' through LTE 120, PT and LTE 220 so that SLAVE CONT 210' controls RLYs(25), (26) and (27) so as to make connection of circuits as shown in FIG. 3 (RLYs(15, 16, 25, 26 and 27));
after that, MASTER CONT 150 polls SLAVE CONT 210' whether PT provided between DI STN 200 and the succeeding DI STN can be used for transmitting a users signal, polling whether a disconnection trouble occurs on the PT (PT OK?);
when the PT must be used for users signal because of the disconnection trouble (PT OK?--NO), MASTER CONT 150 asks whether the succeeding station is another END STN (END STN?);
if the answer in response to (END STN?) is YES, MASTER CONT 150 has no necessity to do any control (END);
when PT is not necessary to be used for the users signal (YES), MASTER CONT 150 sends a command signal to SLAVE CONT 210' so that SLAVE CONT 210' controls RLY(29) so as to make connection of circuit as shown in FIG. 3 (RLY(29) ON);
MASTER CONT 150 sends a command signal to SLAVE CONT 210' so that SLAVE CONT 210' controls RLY(28) so as to make connection of circuit as shown in FIG. 3 (RLY(28) ON); and
the control to be performed by MASTER CONT 150 is ended (END).
As described in reference to FIG. 4(a), the control due to MASTER CONT 150 in FIG. 3 has been very complex, so that it has taken a lot of switching time when a disconnection trouble occurs in WK. Reducing the switching time is very important, because, if the switching time exceeds a designated time in the transmission system, the exceeding switching time would cause to stop the operation of other communication equipment such as an exchange equipment connected to the transmission system. How to reduce the switching time has been a problem in the protection switching equipment of the transmission system.